Authentication systems commonly require that an individual who requests access to computer systems or facilities provide passwords or other personal information known to the individual, access cards or keys possessed by the individual, or biometric features that uniquely characterize the individual, such as fingerprints and hand geometries, in order to authenticate the requestor. Unfortunately, passwords, personal information, and access cards or keys are often illicitly obtained or copied by impostors. While biometric characteristics are harder to illicitly obtain, copy, or impersonate, it is not impossible to do so, and the options for substituting compromised biometric characteristics with other biometric characteristics are limited.